The present invention relates to a nonaqueous electrolyte secondary battery, and more particularly to a nonaqueous electrolyte secondary battery having improved conservation property and discharge cycle characteristic under a high temperature condition and a method of preparing the same.
In recent years, a portable electric and electronic devices such as note type personal computers and portable telephones have improved, the required power for such portable devices has increased, whilst long operation of the devices is also required. In this circumstance, the requirements for improvement in the properties and characteristics of the chargeable battery have rapidly increased. In the prior art, a nickel-cadmium battery is used as the chargeable battery for portable devices. Recently, however, a nickel-hydrogen battery or a lithium battery have been used in place of the nickel-cadmium battery. Particularly, the lithium batter, has a higher energy density by two or three times than that of the nickel-cadmium battery. There is a lithium battery having a negative electrode using carbon material, wherein principally lithium is present in the ionized state but not in the metal state. In the ionized state, charging and discharging reactions are caused. For this reason, the lithium battery with the carbon negative electrode is so called as a lithium ion secondary battery. This lithium ion secondary battery realizes a high capacity, a high output voltage and a high energy density without raising a problem with short circuits due to dendrite growth of lithium which has been a concern in the prior art.
Lithium cobalt acid is used for a positive electrode of the lithium ion secondary battery. Lithium cobalt acid is advantageous because it easily synthesized and is highly stable due to its layered structure; for which reason the lithium ion secondary battery is superior in discharge cycle characteristics and in conservation properties. However, lithium cobalt acid is scarce and expensive. Further, the use of cobalt may raise a problem with the environment. Cobalt system materials are not suitable in view of the cost for various large size lithium ion secondary batteries for electric automobiles or road leveling. In order to settle this problem, developments for lithium nickel acid and lithium manganate have become active.
Particularly, lithium manganate having a spinel structure was discovered by Thackeray et al. as a useful material for the positive electrode of the battery and reported in Mat. Res. Bull., Vol. 18, p. 461, 1983. The lithium manganate having the spinel structure has been attractive. An estimated amount of lithium manganate deposits is large, for which reason lithium manganate is relatively cheap. Manganese is preferable in view of the environmental protection. It was also reported by Fouchard et al. in Electrochem. Sce. Proceedings Vol. 94-28, pp. 348, 1994 that lithium manganate is thermally stable. This means that use of lithium manganate may improve the safety of the battery.
The use of lithium manganate for the battery causes short cycle lifetime due to remarkable deterioration of the conservation property of the battery and also in the capacity of cycle discharge. In order to settle this problem, investigations have been made in view of synthesis conditions, composition, particle size, and specific surface area.
In Japanese laid-open patent publication No. 5-326023, it is disclosed that Li.sub.1-x Mn.sub.2 O.sub.4 (1.gtoreq..times..gtoreq.0) is used for investigation of dry and wet synthesis.
In Japanese laid-open patent publication No. 8-69790, it is disclosed that the specific surface area of Li.sub.1-x Mn.sub.2 O.sub.4 (1.gtoreq..times..gtoreq.0) is defined in the range of 0.05-5.0 m.sup.2 /g. As results of those investigation, almost sufficient stability thereof can be obtained at atmospheric temperature.
The use of the above described materials for the anode of the battery could not realize a sufficient conservation property and discharge cycle characteristic at a temperature higher than 50.degree. C. At the present stage, if the lithium ion secondary battery is used for the portable telephone which are normally used under not so high temperature conditions, then there is almost no problem in use of the lithium manganate having the spinel structure. In the future, however, it is obvious that the battery is to be used under a higher temperature condition such as in note type personal computer and electric cars. For this reason, the requirement for improvements in cycle characteristic and in conservation property under high temperature conditions, for example, at not less than 50.degree. C. have been on the increase.
In the above circumstances, it had been required to develop a novel nonaqueous electrolyte secondary battery free from the above disadvantages.